This invention relates to a method for the detection of a shaft failure in a turbomachine with the purpose of initiating thereupon an appropriate speed-limiting action, more particularly a rapid fuel shut-off on an aero gas-turbine system, in which a torque-exerting turbine rotor and a torque-recipient unit are connected via the shaft which is to be monitored for failure, said shaft being essentially supported at the ends in at least two roller bearings.
In particular for aero engines, but also for industrial gas turbines for power generation, a variety of methods and devices are known which all have the objective of effectively ensuring a speed limitation if the load applied by the torque-recipient unit is lost. The objective is to avoid an uncontrolled increase in speed until self-destruction of the turbomachine, in particular of combustion turbomachines, and to prevent dangers to persons and property. Such critical operating conditions may occur if the power generator is disconnected from the electrical power-supply system in an uncontrolled manner (loss-of-load), for example in power stations with combustion turbomachines. Similarly, a failure of the shaft between the energy-generating system, i.e. the turbine rotor, and the energy-consuming system, in particular a compressor, may result in an uncontrolled increase in speed of the former. In the case of an aero engine or an aero gas-turbine system, respectively, the energy-consuming or torque-recipient system may be the fan.
In a variety of known Patent Specifications, speed-limiting devices for aero engines are described in which, upon a failure of the shaft between the energy-consuming section (e.g. the compressor) and the energy-generating section (e.g. the turbine rotor), a mechanical principle of action is applied giving way to an axial relative movement, and ultimately to the collision, of the stator and the blades of the turbine rotor. In the process of collision (also termed xe2x80x9ctanglingxe2x80x9d), the rotational energy of the turbine rotor is dissipated by deformation, friction and destruction of the turbine rotor and stator blading concerned until standstill. For this principle of action, reference is made to the Patent Specifications U.S. Pat. No. 4,505,104, U.S. Pat. No. 4,503,667 and U.S. Pat. No. 4,498,291, for example.
In a further mechanical solution for the control of the overspeed of lower-output aero engines upon failure of the drive shaft between the low-pressure turbine and the fan, the drive shaft between the low-pressure turbine and the fan is provided with a reference shaft. In the case of a drive shaft failure, the failed drive shaft and the reference shaft will change their relative positions. A pre-loaded follower will be released and engage a wire loop. Since the low-pressure turbine continues to rotate, a pull will be exerted on the wire loop, which initiates a rapid shut-off of the fuel via a cable.
As regards an electronic solution of the overspeed problem, Patent Specification U.S. Pat. No. 4,474,013 teaches a circuitry for a steam turbine. This solution uses up to four speed sensors that operate redundantly and are associated with a gear shaft. The resultant signals of the speed sensors are proportional to the speed of the gear shaft. An appropriately designed electronic measuring-data system differentiates the speed signal and produces a derivative of acceleration. The series-connected fresh-steam valves (a stop valve and a control valve) are actuated in a pre-set overspeed situation by the acceleration values determined being processed as well as upon transgression of a speed threshold.
A further electronic solution of the overspeed problem for an aero gas-turbine system is described in Patent Specification U.S. Pat. No. 4,712,372. Two sensors are arranged on the toothed turbine shaft which produce a signal that is speed-proportional to the number of teeth of the shaft. Both sensors operate redundantly with each other, with the one channel being analog and the other channel providing digital signal processing and transmission. If an overspeed situation is detected by both sensors, a solenoid fuel valve will be actuated and the fuel supply interrupted.
Patent Specification U.S. Pat. No. 4,635,209 teaches another electronic solution for controlling overspeed situations in connection with a steam turbine. In this solution, the principle of measurement is again based on a pulsed measuring signal produced on a toothed shaft. To enhance the measured value accuracy, three independent measuring channels are used at the same measuring location. One of the three measuring channels is provided with a monitoring function. Each of the measuring channels communicates via a programmable computer.
Accordingly, the known or published systems for the control and limitation of overspeed conditions are either of the mechanical or the electromechanical/electronic type.
A commercial embarrassment to the aforesaid problem solution, therefore, lies in the plurality of the systems which, in terms of design, are to be adapted to the specific conditions of the respective aero engine. In the case of aero engines that apply the tangling principle to safely control a shaft failure between the fan and the low-pressure turbine, total loss of the blading and correspondingly high replacement costs are to be anticipated. A mechanical system using a reference shaft will, upon actuation, lose at least part of the components and, also, increase the mass of the engine, a circumstance which is apparently undesirable for aerospace applications.
Accordingly, the mass-cost relation of mechanical solutions for the implementation of the required safety shut-off function upon failure of the shaft between the fan and the low-pressure turbine is to be considered to be adverse with regard to manufacturing and operating costs. Electromechanical or electronic solutions are clearly outdistancing the mechanical solutions in terms of total costs.
The known electromechanical and electronic solutions are applied solely for the monitoring of a specified rotor speed. These systems are presently not capable of detecting shaft failures. In particular aero gas turbines in the higher performance classes and turbines of industrial power plants, for which light-weight construction is irrelevant, have a moment of inertia the magnitude of which is commensurate with the time necessary to counteract overspeed with the conventional electromechanical and electronic methods (speed measurement process and actuators) and the associated high dead times and time lags. Speed measurement processes used in these applications are based on the summation of discrete individual pulses over a measuring period. The known electromechanical and electronic solutions are considered technically inappropriate for lower-performance aero engines, since, in combustion turbomachines with very low moments of inertia, these solutions do not respond fast enough to a demand case. In the case of smaller engines, therefore, the required measuring period is too large in relation to the time that is left to detect a shaft failure, generate the required actuating signal and actuate the rapid shut-off.
Further, the known measuring devices for rotational speed and their derivatives, such as angular velocity and angular acceleration, have insufficient sensitivity and measuring resolution to produce a measuring signal in the short time necessary for the actuation of rapid shut-off and speed limitation.
In a broad aspect, the present invention provides an accordingly improved, in particular cost-effective and safe method for the detection of a shaft failure on a turbomachine.
As a particular object of the present invention, the rotational frequencies of the two shaft ends in the respective roller bearings of the shaft to be monitored for failure are determined and compared with each other continually and essentially in real time, and a shaft failure is inferred if the rotational frequency on the roller bearing on the side of the turbine rotor exceeds the rotational frequency on the roller bearing of the torque-recipient unit.
Further objects and advantages are cited in the subclaims, in particular beneficial features of a preferred apparatus for the implementation of the method in accordance with the present invention.
The present invention preferably refers to the problem of a failure of the shaft between the fan as torque-recipient unit and the torque-exerting low-pressure turbine rotor of an aero engine or an aero gas-turbine system, respectively, and to the required limitation of the speed of the low-pressure rotor, but may be applied similarly to any turbomachinery. The object here is to provide an electromechanical/electronic embodiment of the said method and the respective apparatus.
In accordance with the present invention, the rotational frequency of each end of a shaft of a turbomachine which is essentially supported at the ends in roller bearings is determined in the respective roller bearing. If significant differences between the rotational frequencies of the two shaft ends are encountered, failure of the shaft will be inferred and, consequently, an appropriate speed-limiting action will be initiated.
While this proposal may appear relatively simple at first glance, the requirements imposed on measurement techniques and the pertinent evaluation electronics are extremely stringent to ensure the required level of safety, for example for aero engines. The entire process for the determination of the rotational frequency must accordingly be executed extremely fast, i.e. the determination of the rotational frequencies and the subsequent evaluation should be accomplished in real time to respond as rapidly as possible to a shaft failure so detected. In a preferential arrangement, therefore, a separately operating measuring channel is provided for each roller bearing to determine the rotational frequency of the respective shaft end in the roller bearing, with both measuring channels being connected to a comparator for the purpose of comparison of the rotational frequencies and with the generation of the measuring signal, its transmission and processing until comparison of both rotational frequencies being accomplished in the real-time frame. Accordingly, if a significant difference between the two rotational frequency occurs, an electric variable can then be generated in real time to initiate an appropriate speed-limiting action, for example the closure of a fuel quick-action shut-off valve.